The present invention relates generally to horn antennas, and, more particularly, to horn antennas capable of operating at two or more separate frequencies and capable of providing equalized E and H plane patterns at each of the frequencies.
In the communication field, a number of systems exist which require antenna systems to be capable of operating at two or more separate frequencies. For example, in military and commercial satellite systems, it is common for the uplink signal from a ground station to the satellite to have a first frequency while the downlink signal from the satellite to the ground station has a second frequency. Commercial and military Ka-Band communication satellites are one example of this where the uplink frequency is 20 GHz and the downlink frequency is 30 GHz.
In the past, communication satellite systems such as those mentioned above have handled the two frequencies by using reflector antenna systems in the satellite which are designed with an antenna feed (for example, a feed horn) and a reflector system (generally using a primary reflector and a sub-reflector). In such an arrangement, separate horn antennas are often used as the feeds, with one horn antenna provided for each frequency to be covered. On the other hand, various systems have been developed using a single horn operating at dual frequencies. U.S. Pat. No. 3,938,159, U.S. Pat. No. 4,785,306 and U.S. Pat. No. 5,003,321 are three examples of such dual frequency feed horns that can be used in a satellite communication system. However, these arrangements are somewhat complicated to construct, and are not readily adaptable to equalizing the E and the H plane patterns at the different frequencies.
In their studies, the inventors considered the possibility of using a corrugated horn operating at two or more separate frequencies such as the above-noted 20 GHz and 30 GHz frequencies in the Ka-Band. Corrugated horns (i.e., horns where corrugated recesses are provided which each have a depth extending radially to the central axis of the horn) have an advantage in being able to readily provide antenna patterns that are equal in the E and H planes by effectively terminating substantially all of the current parallel to the inner wall of the horn (so that the horn will have the same boundary conditions that exist for the E field perpendicular to the wall). To this end, the inventors designed and studied a corrugated horn such as shown in FIG. 1.
In the arrangement shown in FIG. 1, a corrugated horn 10 has a plurality of corrugated recesses 12 that gradually increase in depth and width from an inner portion of the horn to an outer portion. By virtue of the different depths, the center frequency of each of the recesses 12 will be slightly different than that of the adjacent recess 12. Typically, the depth is set at .lambda./4 to tune to the desired frequency. The width of each corrugation recess 12 determines the bandwidth of that particular recess around the center frequency. Thus, by properly designing the depth and the width of each of the recesses 12, the horn of FIG. 1 can provide continuous coverage of a desired frequency band. Also, by properly setting the depth and width of the corrugation recesses, equalized E and H plane patterns can be provided within that frequency band, as noted above.
In further considering this structure, the inventors studied the possibility of providing two or more groups of corrugation recesses 12 in a horn such as FIG. 1, to thereby construct a horn which would operate at two distinct frequency bands (e.g., centered around 20 GHz and 30 GHz, for example), while providing equalized E and H plane patterns at each of these separate frequency bands. However, after considering this, the inventors noted a fundamental problem which would exist with such an arrangement. Specifically, as shown in FIG. 1, the electrical aperture of the corrugated horn 10 would be limited to the inner diameter of the horn. Because of the corrugation recess construction, this inner diameter will be substantially smaller than the actual maximum physical diameter of the horn. In other words, the corrugated horn 10 of FIG. 1 has a significantly larger physical aperture than its electrical aperture. This can be a serious drawback, particularly in terms of size and weight considerations which are involved in construction of a satellite antenna. Also, the relatively large physical diameter of such a horn could serve as a significant constraint in reflector systems used in satellites wherein a plurality of feed horns might be located adjacent to one another to provide multiple coverage beams from a single reflector system.